Magnetically directed self-assembly of molecular electronic junctions

ABSTRACT

A device having a substrate, a pair of ferromagnetic leads on a surface of the substrate, laterally separated by a gap, and one or more ferromagnetic microparticles comprising a conductive coating at least partially within the gap. The conductive coating forms at least part of an electrical connection between the leads. A molecular junction may connect the leads to the microparticle.

This application is a divisional application of U.S. patent applicationSer. No. 11/044,197, allowed, filed on Jan. 28, 2005, incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a molecular electronic junction.

2. Description of the Prior Art

Electronic devices based on the electronic characteristics of smallassemblies of active molecules (commonly referred to as molecularelectronics) have the potential of exceeding the expected scaling limitsforeseen by many as a looming barrier to Moore's Law.

A formidable challenge to the realization of molecular electronics isthe development of high-throughput techniques for assemblingnanoelectronic devices on a scale competitive with establishedcomplementary metal-oxide-semiconductor (CMOS) processes. Also neededare reliable methods capable of addressing each nanoscale unit withelectrodes in such a way as to not alter or damage the active speciesduring the assembly process. Therefore, an important step towardspractical molecular electronic device fabrication is the development ofa “soft” self-assembly processes which combine high yield with parallelassembly and establish contact between all nanostructures and patternedelectrodes in a process which does not damage the active molecular unit.

SUMMARY OF THE INVENTION

The invention comprises a device comprising a substrate, a pair offerromagnetic leads on a surface of the substrate, laterally separatedby a gap, and one or more ferromagnetic microparticles comprising aconductive coating at least partially within the gap. The conductivecoating forms at least part of an electrical connection between theleads.

The invention further comprises a method of making a device comprisingthe steps of: providing a substrate comprising a pair of ferromagneticleads on a surface of the substrate, laterally separated by a gap;providing a dispersion of one or more ferromagnetic microparticlescomprising a conductive coating; contacting the dispersion to the gap;and applying a magnetic field to the substrate, whereby the one or moremicroparticles are deposited in the gap to form at least part of anelectrical connection between the leads.

The invention further comprises a particle comprising a microsphere, aferromagnetic coating on the microsphere, and a conductive coating onthe ferromagnetic coating. The ferromagnetic coating and the conductivecoating cover only one hemisphere of the microsphere.

The invention further comprises a method of making a particle comprisingthe steps of: proving a plurality of microspheres; forming aclose-packed monolayer of the microspheres on a substrate; placing aferromagnetic coating on the hemispheres of the microspheres facing awayfrom the substrate; and placing a conductive coating on theferromagnetic coating.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Example Embodiments andthe accompanying drawings.

FIG. 1 schematically shows a process for making the device of theinvention.

FIG. 2 shows a scanning electron microscopy (SEM) micrograph of aproperly aligned microsphere in a gap.

FIG. 3 schematically illustrates molecular junctions to the microsphere.

FIG. 4 shows a current-voltage (I-V) plot of three discrete microspherejunctions.

FIG. 5 shows I-V analysis of oligo(phenylene ethynylene) dithiol (OPE)and oligo(phenylene vinylene) dithiol (OPV) single-bead microspherejunctions.

FIG. 6 shows I-V characteristics of OPE acquired using scanningtunneling microscopy (STM), microsphere, and crossed-wire test beds.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present invention. However, it will beapparent to one skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well-known methods anddevices are omitted so as to not obscure the description of the presentinvention with unnecessary detail.

A technique is disclosed for fabricating molecular junctions based onmagnetic directed assembly of metallized silica beads. Magneticentrapment exploits micron to nanoscale magnetic fields induced atengineered focal points within a ferromagnetic array to direct thedeposition of susceptible species to the high-field region generatedbetween pre-defined contacts. Magnetically-driven self-assembly canimpart desirable properties to device fabrication such as high yield,accurate placement, and predictable orientation for deposited species.Magnetic entrapment can be used for the controlled deposition ofmetallized silica colloid into features functionalized withself-assembled monolayers (SAMs). Well-defined species which areinherently non-responsive towards magnetic entrapment can be made toundergo this simple process of self-assembly and form reliable molecularjunctions using a non-aggressive technique which does not damage theorganic monolayer.

Fabrication of reliable molecular junctions via magnetic assembly can befacilitated by deposited species that are attracted to regions oflocally intense magnetic fields, electrically conductive, and nearlyidentical in size and shape.

FIG. 1 illustrates a procedure used to fabricate and magneticallydeposit the microspheres. In one step of the process of making thedevice, a substrate comprising a pair of ferromagnetic leads on asurface of the substrate, laterally separated by a gap is provided. Sucha substrate 10 is shown in FIG. 1. An example substrate is silicon. Anexample leads is permalloy metal with a gold coating. The gap may be,but is not limited to, no more than about 5 μm wide. Methods of placingsuch leads on a substrate are known in the art, including, but notlimited to, lithography.

There can also be a plurality of pairs of leads on the substrate. Suchpairs may be arranged in a grid. High aspect-ratio features promotealignment of magnetic domains parallel with the long axis of each deviceand minimizes flux-closed domain configurations which reduce overallfield strength within the magnetic trap.

In another step of the process, a dispersion of one or moreferromagnetic microparticles comprising a conductive coating isprovided. The dispersion is a means for delivering the microparticles tothe leads and may be prepared by sonicating the microparticles in aliquid. The microparticles may be larger, including slightly larger thanthe width of the gap.

An example particle has three parts: a microspherical core, which may besilica; a ferromagnetic coating, which may be nickel, on the core; and aconductive coating, which may be gold, on the ferromagnetic coating. Thesilica may be colloidal silica of no more than 5 μm in diameter. Silicacolloid may be used for this process as they are inert, available in abroad range of diameters, and obtained with narrow size distributions.The term “microsphere” also includes smaller particles such asnanospheres. The coatings may cover only a single hemisphere of thecore. This is the natural result of a certain method of making suchmicroparticles. Such particles may be made by forming a hexagonalclose-packed monolayer of the microspheres on a substrate, then placingthe ferromagnetic and conductive coatings on the microspheres. Thecoatings are formed on only the hemisphere facing away from thesubstrate.

Fabrication of electrically conductive and magnetically susceptiblesilica colloid may be accomplished through evaporative metallizationtechniques. The hemispherical-metallization of microspheres has beenpreviously demonstrated by the physical evaporation of gold ontohexagonal close-packed arrays of colloid monolayers. (Love et al., NanoLett., 2, 891 (2002); Choi et al., Nano Lett., 3, 995 (2003). Allreferenced publications and patents are incorporated herein byreference.) The ferromagnetic layer of nickel provides the magnetichandle needed to manipulate the resulting colloid from a solution-basedsuspension while the thin gold overcoat yields an oxide-fi-ee surfacefor charge transport into an organic monolayer.

In this technique, the silica sphere acts as an inert template for themetal layers conforming to their surfaces permitting the fabrication ofnearly identical magnetic and electrically conductive building blocksfor magnetic assembly. This method may be extended to include a numberof inert species which are uniform in size and shape in order to renderthem magnetically susceptible for controlled deposition into a proximalprobe test-bed.

Once the substrate and dispersion are provided, the dispersion iscontacted to the gap between the leads. The dispersion may also contactother parts of the substrate. This may be performed as simply asimmersing the substrate in the dispersion, or placing a drop or puddleof the dispersion on the substrate.

While the dispersion is in contact with the gap, a magnetic field isapplied to the substrate. The field can cause one or more microparticlesto be deposited in the gap. When using half-coated microspheres, thefield lends to orient the sphere with the conductive coating contactingboth leads to form at least part of an electrical connection between theleads. Other kinds of microparticles may not need such orientation,however, the magnetic field in combination with the ferromagneticproperty of the leads still serves to place the microparticles into thegap. Deposition of the microparticles may be more efficient when themagnetic field is approximately parallel to the leads. An examplemagnetic field strength in 100 G, however, different systems may requiredifferent alignment field strengths.

By magnetically controlling the face of the colloid which points intoeach device, this technique is able to promote electrical contact acrosssource and drain electrodes despite half of the bead beingnon-conductive. Control over orientation of the sphere is expected tooccur via magnetic interactions between the underlying ferromagneticlayer, the externally applied magnetic field, and the localizedmagnetism being generated in each device. FIG. 2 shows an SEM micrographof a properly aligned microsphere in a gap.

When the microparticles are larger than the gap, it is possible toachieve a high yield of devices having only a single microparticlespanning the gap, in contact with both leads. This allows forconsistency among multiple devices.

The resulting device, which may be made by other methods, can optionallyinclude electrically active molecules. Such molecules may have afunction as part of a circuit. The molecules of a monolayer in contactwith a lead and the microparticle. This may be made by the presence of amonolayer of the molecules on the lead before the microparticle isdeposited into the gap. This allows for a molecular junction to beformed across a junction that is longer than the molecules themselves.

In one embodiment, both leads are coated with the molecules, which arein contact with a single particle in the gap. The molecules may compriseone or more sulfur atoms bound to a gold coating on the microparticle,the leads, or both. Suitable molecules include, but are not limited to,a compound having conjugated bonds, 1-undecanethiol, oligo(phenylenevinylene) dithiol, and oligo(phenylene ethynylene) dithiol. In anotherembodiment the microparticles are coated with the molecules.

Similar to previous methods of particle entrapment for charge transportanalysis in organic films, the method can produce two molecular junctionis in series as illustrated in FIG. 3. However, the use of magneticentrapment offers in addition to this a controlled “bottom-up”self-assembly process which does not require alternating electricfields, individually addressing devices to initiate deposition, e-beamlithography, or high temperatures. Since all potential molecularcandidates for memory applications include chemical functionality andbond structure important for specific electronic responses, the use ofmagnetic entrapment to assemble such devices becomes attractive due toits ability to accurately place conductive units in intimate contactwith organic monolayers without using harsh chemicals or processes whichmay erode or alter the SAM. Also, since magnetic entrapment performswell as a parallel technique this method has the potential to generate alarge number of devices simultaneously in a wafer-level assemblyprocess. Due to the small contact area achieved through the use ofrelatively large micron-scale spherical beads, this method can provide asimple means to fabricate nanoscale devices using high-throughputoptical lithography.

Having described the invention, the following examples are given toillustrate specific applications of the invention. These specificexamples are not intended to limit the scope of the invention describedin this application.

Example 1

Formation of microspheres—2-D close-packed hexagonal monolayers of 1.5micron silica beads were formed on 100 mm diameter silicon wafers by theslow evaporation of an ethanol dispersion of the colloid (BangsLaboratories). Three prepared wafers were mounted simultaneously in anevaporative metallization chamber. 50 nm of nickel was deposited evenlyon all wafers followed by 10 nm of gold. Following metallization, 100 mLof absolute ethanol was placed in a large beaker and the prepared waferswere added individually. Mild sonication for 20 min in a bath sonicatorcompletely removed all metallized beads from the silicon surfacecreating a dark grey ethanol dispersion of the beads. All three waferswere stripped of their beads using the same dispersion. Thisconcentrated “stock” solution was sonicated for another 20 minutesfollowed by removal of 0.5 mL of the dark dispersion. This aliquot wasdiluted to 100 mL with ethanol and again sonicated for 30 minutes.Approximately 10 mL of this dilution was used for each run ofmicrosphere deposition after briefly sonicating.

Analysis by scanning electron microscopy (SEM) confirmed the hemisphereof the colloid which faced into the deposition was covered with a thindeposit of metal while the “underside” remained uncoated. In an SEMimage of one of these prepared 1.5 μm diameter metallized silicaspheres, the metallized and non-metallized regions were plainly visibleand found to occupy distinct hemispheres on the sphere.

Example 2

Magnetic self-assembly—The magnetic self-assembly of metallized colloidwas conducted using arrays of high-aspect ratio devices composed ofelectroplated permalloy metal (Ni₈₀Fe₂₀), the fabrication of which isdescribed in Long et al., Adv. Mater., 16, 814 (2004). Features wereencased in a thin layer of electroless gold to permit thiol monolayerformation on the surface of the electrodes. Each device incorporated aset of probe tips separated by a 1 μm gap where a locally intensemagnetic field would direct entrapment of the microspheres. The specificcombination of a 1 μm gap width and 1.5 μm diameter colloid wascarefully chosen to prevent the spheres from fitting properly in thegaps. This forces electrical contact to be made further into themetallized region of the sphere where the deposited Ni/Au film is morecontinuous.

Approximately 3 ml of freshly sonicated microsphere dispersion wasplaced in a small test tube. The test tube was then placed in a 100Gauss magnetic field generated by two rare-earth magnets at a fixeddistance from each other. Substrates containing prepared device arrayswere dropped into the test tube and a timer started. The tube was turnedto ensure the long axis of the permalloy devices was parallel with theexternal alignment field and trapping was allowed to proceed for 7.0minutes. The tube was then removed from the magnetic field and the beadsolution poured out. The substrate was then rinsed several times withethanol to remove excess colloid from the surface. Substrates were thengently dried with a stream of carbon dioxide or nitrogen and placedunder vacuum until electrical analysis was performed.

Trapping conditions were optimized for the arrays and specific colloidconcentration. Best results were obtained when the ferromagnetic core ofeach device was additionally aligned using an external magnetic field(100 Gauss) during the trapping process. Trapping was performed for 7minutes followed by rinsing with ethanol and drying each substratebefore analysis. Efficiency for single-bead entrapment was found to beapproximately 60%, while 20-30% resulted in 2-3 beads per device, and10-20% yielded no beads. When single-bead junctions were not required, ayield of 100% could be readily obtained by increasing trapping time butresulted in multiple beads per device. SEM confirmed the metallizedhemisphere of the trapped colloid was oriented preferentially towardsthe high-field region induced between the electrodes.

Example 3

Current-voltage analysis—Room temperature current-voltage (I-V) analysiswas conducted on non-functionalized microsphere junctions to determinethe electrical properties of the metal-metal contacts generated usingthis technique. Improper bead orientation after trapping preventselectrical current through the device, however the majority of trappedbeads display a preferred magnetic orientation which promotes electricalcontact. Proper alignment of the colloid completes the circuit acrossthe probe tips. I-V analysis of several such junctions revealed contactresistance from 100-600Ω, similar to reports of assembled metallicnanowires. The instrument compliance was set at 1 μA. The range ofmicrosphere junction resistance is thought to be due to the exactpositioning of the sphere and its manner of contact with the electrodes.Electrical contacts made near the edge of the metallized and silicaregions were found to be of higher resistance due to lower thickness ofthe deposited metal film.

Example 4

Deposition of monolayers—Substrates containing gold-coated permalloyfeatures were cleaned using 30% H₂O₂ for 30 minutes followed bydeionized water and ethanol rinses. Substrates were then exposed toUV/O₃ treatment for 10 minutes and thoroughly rinsed again in ethanoland dried in a stream of nitrogen. A self-assembled monolayer of C-11,OPE, or OPV (shown below) were then deposited on the gold-coatedelectrodes by immersion for 24-48 hours in a 1 mM solution of thecompound in either ethanol or tetrahydrofuran. Monolayers of OPE and OPVwere deposited as the dithiolacetates and base-deprotected immediatelybefore SAM formation using ammonium hydroxide. OPE and OPV were handledand deposited under the inert atmosphere of a nitrogen glove box. Afterself-assembly of the monolayer, the substrates were rinsed with ethanoland dried in nitrogen/vacuum.

Example 5

Current-voltage analysis with monolayers—The I/V characteristics ofmicrosphere junctions fabricated using electrode arrays functionalizedwith various classes of self-assembled monolayers were examined. Thespecific molecules are shown above and represent species ranging fromelectrical insulators to prototypical molecular wires with extensiveπ-conjugated systems. FIG. 4 shows a plot of three discrete microspherejunctions incorporating 1-undecanethiol (C-11) monolayers. Single-beadmicrosphere junctions were found to yield room temperaturecurrent-voltage measurements indicative of electron tunneling withconductance ranging from 0.5-1.0 nA at 1 V bias. Zero-bias resistancewas calculated from the slope of the linear I-V segment between ±0.1 Vand averaged 3.5×10⁹Ω (3.5 GΩ). Junctions were generally stable up to ±2V but would deteriorate rapidly at increasing bias. Comparison of theseresults to previous measurements on alkanethiol systems allowed forestimating of the number of C-11 molecules the microsphere junctionsincorporate. Based on reported C-11 conductance of 1.3 pA/molecule at0.5 V, it is estimated that 1.5 micron diameter microspheres addressapproximately 300 C-11 molecules per contact (two contacts per sphere).These values scale appropriately to results from 10 μm diametercrossed-wire test-beds reported to address ˜1000 molecular units. Sincemicrosphere junction area is dependant on the radius and curvature ofthe sphere trapped, use of smaller silica colloid could be used to lowerthe contact area achieved with this technique and further reduce thenumber of molecules analyzed. The entrapment of multiple beads perdevice (usually no more than 2-3) occasionally led to the formation ofparallel molecular junctions. In these instances SEM was used todetermine the number of successful contacts made by the beads andcompared to I-V characterization of the junction. In parallelmicrosphere junctions conductance increased linearly with the number ofcolloids making contact across the electrodes.

The electrical properties of microsphere junctions incorporatingmonolayers of oligo(phenylene ethynylene) dithiol (OPE) andoligo(phenylene vinylene) dithiol (OPV) were examined. Unlikealkanethiols, the unsaturated backbone of oligo(phenylene) derivativescontain π-conjugation which extends between the thiol end groups of eachmolecule yielding electron transport properties typical of organicconductors. Monolayers of OPE and OPV were deposited as thedithiolacetates and base-deprotected using ammonium hydroxideimmediately before SAM formation. Microsphere junctions weremagnetically self-assembled as before (100 G/7 min.) followed by vacuumannealing to promote thiol-gold bond formation on the Au/Ni hemisphereof the trapped colloid and minimize junction resistance. FIG. 5 showsI-V analysis of OPE and OPV single-bead microsphere junctions plottedwith C-11 data from FIG. 4 for comparison. The overall trend inconductance follows C-11<<OPE<OPV as expected for an insulatingalkanethiol verses π-conjugated molecules. Junctions incorporating theπ-conjugated OPE and OPV monolayers were found to conduct nearly twoorders of magnitude more current than those containing the insulatingalkanethiol. OPE microsphere junctions conduct approximately 40 nA at 1V bias and were calculated to have a zero-bias resistance (±0.1 V) of33.4 MΩ. Typical junctions incorporating monolayers of OPV conductedapproximately 4× as much current as OPE and averaged a zero-biasresistance of approximately 10.3 MΩ. The trends observed here are inexcellent agreement with previous measurements on these classes ofmolecules using crossed-wire test-beds, scanning tunneling microscopy(STM) experiments, nanowire-based molecular junctions, and theoreticalstudies. The consistency of results found between these past reports andcurrent microsphere molecular junctions validate this new technique asuseful for molecular transport measurements.

Another evaluation of microsphere molecular junctions was performed bycomparing the single-bead OPE results to those of previous techniques inorder to properly place this system in perspective with establishedmethods. FIG. 6 shows I-V characteristics of OPE acquired using STM,microsphere, and crossed-wire test beds. On the lower end of thelogarithmic scale STM results are shown for an individual OPE moleculeisolated in a matrix of insulating SAM of alkanethiol. Three orders ofmagnitude above the individual OPE curve lies data from a crossed-wireexperiment known to sample approximately 1000 molecules. Results from anOPE microsphere junction is found to lie intermediate between these twomethods. From this comparison, and a similar one using OPV data, themicrosphere junctions are calculated to incorporate approximately 60-100OPE and OPV molecules. When compared to the independent estimate for thenumber of C-11 molecules (150-300) fewer molecules are found within theOPE/OPV junctions as compared to their alkanethiol counterparts. Weattribute this decrease to lower packing density of the r-conjugatedspecies. From the combined C-11, OPE, and OPV data, it is estimated that1.5 μm diameter microsphere junctions are in contact with 100-300individual molecules covering approximately 60 mm area on the magneticelectrodes.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that the claimed invention may be practiced otherwise than asspecifically described.

1. A particle comprising a microsphere; a ferromagnetic coating on themicrosphere; and a conductive coating on the ferromagnetic coating;wherein the ferromagnetic coating and the conductive coating cover onlyone hemisphere of the microsphere.
 2. The particle of claim 1, whereinthe microsphere comprises silica; wherein the ferromagnetic coatingcomprises nickel; and wherein the conductive coating comprises gold. 3.A method of making a particle comprising the steps of: proving aplurality of microspheres; forming a close-packed monolayer of themicrospheres on a substrate; placing a ferromagnetic coating on thehemispheres of the microspheres facing away from the substrate; andplacing a conductive coating on the ferromagnetic coating.